Place fillets on a horizontal surface. With a sharp knife open each fillet lengthwise (like a book). Place fillets over plastic film on a horizontal surface, overlapping slightly.
Place another plastic film over and pound fillets carefully forming a rectangle. Season with salt and pepper.
Remove film and spread mustard on fillets.
Heat oil in a skillet and sauté onions, carrots, and leeks until just cooked.
Arrange vegetables in the center of the rectangle, lengthwise, and place prunes on top.
Roll chicken, lifting plastic film to help and tie with a string.
Place chicken roll in a ovenproof pan and pour stock over it.
Bake in oven 350 °F (180 °C) for 45 minutes or until tender.
Remove string and cool lightly. Cut in slices.